One aspect of the invention relates to liquid-crystalline mixtures for twisted nematic (TN) and supertwisted nematic (STN) liquid-crystal displays having very short response times and good steepnesses and angle dependencies. The mixtures according to the invention are furthermore also suitable for IPS (in plane switching) displays.
TN displays are known, for example from M. Schadt and W. Helfrich, Appl. Phys. Lett., 18, 127 (1971). STN displays are known, for example from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals (8.-10.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987) and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term STN here covers any relatively highly twisted display element having a twist angle with a value of between 160xc2x0 and 360xc2x0, such as, for example, the display elements according to Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs (EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
STN displays are distinguished from standard TN displays by significantly better steepnesses of the electro-optical characteristic line and, at moderate and relatively high multiplex rates, for example from 32 to 64, by better contrast values. On the other hand, the contrast in TN displays is generally higher, owing to the better dark value, and the angle dependence of the contrast is lower than in STN displays having low multiplex rates of, for example, less than 32.
Of particular interest are TN and STN displays having very short response times, in particular at relatively low temperatures. In order to achieve short response times, the rotational viscosities of the liquid-crystal mixtures have hitherto been optimised using mostly monotropic additives having relatively high vapor pressure. However, the response times achieved were not adequate for every application.
In order to achieve a steep electro-optical characteristic line in the displays according to the invention, the liquid-crystal mixtures should have relatively large values for the ratio between the elastic constants K33/K11 and relatively small values for xcex94xcex5/xcex5xe2x8axa5, where xcex94xcex5 is the dielectric anisotropy and xcex5xe2x8axa5 is the dielectric constant perpendicular to the longitudinal molecular axis.
In addition to optimisation of the contrast and response times, further important requirements are made of mixtures of this type:
1. broad d/p window
2. high long-term chemical stability
3. high electrical resistance
4. low frequency and temperature dependence of the threshold voltage.
The parameter combinations achieved in the art are still far from adequate, in particular for high-multiplex STN displays (with a multiplex rate in the region of about {fraction (1/400)}), but also for medium- and low-multiplex STN displays (with multiplex rates in the region of about {fraction (1/64)} and {fraction (1/16)} respectively), and TN displays. This is partly attributable to the fact that the various requirements are affected in opposite manners by material parameters.
Thus, there continues to be a great demand for TN and STN displays, in particular for moderate- and low-multiplex STN displays, having very short response times at the same time as a large working-temperature range, high characteristic-line steepness, good angle dependence of the contrast and low threshold voltage which meet the above-mentioned requirements.
One of the objects of the invention is to provide TN and STN displays which do not have the above-mentioned disadvantages or do so only to a lesser extent, and at the same time have short response times, in particular at low temperatures, and very good steepnesses.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
These objects can be achieved by using nematic liquid-crystal mixtures which comprise one or more compounds of formula IA, 
one or more compounds of formula IB, 
xe2x80x83and
one or more compounds of formula IC, 
wherein
Ra, Rb and Rd are each, independently of one another, an alkyl group having up to 12 carbon atoms which is unsubstituted or substituted by at least one halogen atom and in which one or two non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94COxe2x80x94Oxe2x80x94 in such a way that O atoms are not linked directly to one another, where, in the case where a=1, Rd may alternatively be F, Cl, OCHFCF3, CF3 or OCF3,
Rc is alkenyl or alkenyloxy having from 2 to 6 carbon atoms, 
Z is xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94 or a single bond,
L1 to L4 are each, independently of one another, H or F, and
a is 0 or 1,
where the proportion of compounds of formula IC in the mixture is at least 25% by weight.
The use of the compounds of the formulae IA, IB and IC in the mixtures for TN and STN displays according to the invention results in at least
high steepness of the electro-optical characteristic line,
low temperature dependence of the threshold voltage, and
very fast response times, in particular at low temperatures.
The compounds of the formulae IA, IB and IC significantly shorten, in particular, the response times of TN and STN mixtures while simultaneously increasing the steepness and low temperature dependence of the threshold voltage.
The mixtures according to the invention are furthermore distinguished by at least the following advantages:
they have low viscosity,
they have low threshold voltage and operating voltage, and
they effect long shelf lives in the LC display at low temperatures.
The mixtures according to the invention are also suitable for IPS displays.
The mixtures according to the invention are, in particular, suitable for liquid-crystal displays having
two outer plates, which, together with a frame, form a cell,
a nematic liquid-crystal mixture of positive dielectric anisotropy located in the cell,
electrode layers with alignment layers on the insides of the outer plates,
a tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0 degree to 30 degrees, and
a twist angle of the liquid-crystal mixture in the cell from alignment layer to alignment layer with a value of between 22.5xc2x0 and 600xc2x0.
Liquid-crystal displays of this type preferably contain a nematic liquid-crystal mixture comprising
a) 15-80% by weight of a liquid-crystalline component A containing one or more compounds having a dielectric anisotropy of greater than +1.5;
b) 25-85% by weight of a liquid-crystalline component B containing one or more compounds having a dielectric anisotropy of between xe2x88x921.5 and +1.5;
c) 0-20% by weight of a liquid-crystalline component D containing one or more compounds having a dielectric anisotropy of below xe2x88x921.5, and
d) if desired, an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is greater than about 0.2, preferably 0.2 to 1.3,
characterised in that they comprise at least one compound of formula IA, 
one or more compounds of formula IB, 
xe2x80x83and
one or more compounds of formula IC, 
wherein
Ra, Rb and Rd are each, independently of one another, an alkyl group having up to 12 carbon atoms which is unsubstituted or substituted by at least one halogen atom and in which one or two non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94COxe2x80x94Oxe2x80x94 in such a way that O atoms are not linked directly to one another, where, in the case where a=1, Rd may alternatively be F, Cl, OCHFCF3, CF3 or OCF3,
Rc is alkenyl or alkenyloxy having from 2 to 6 carbon atoms, 
Z is xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94 or a single bond,
L1 to L4 are each, independently of one another, H or F, and
a is 0 or 1,
where the proportion of compounds of the formula IC in the mixture is at least 25% by weight.
The invention also relates to TN and STN displays, in particular moderate- and low-multiplexed STN displays.
Preference is given to liquid-crystal mixtures which comprise one or more compounds of the formula IA in which Ra is a straight-chain alkyl group having from 1 to 8 carbon atoms or an alkenyl group having 2-8 carbon atoms.
Preference is furthermore given to liquid-crystal mixtures which comprise one or more compounds of formula IB in which Rb is a straight-chain alkyl or alkenyl group having from 1 or 2 to 8 carbon atoms respectively.
The mixtures according to the invention comprise, in particular, one or more, preferably one or two, compounds of the sub-formulae IA-1 to IA-4: 
wherein Ra is as defined above.
Preferred compounds of the formula IA-1 are those selected from the group consisting of IA-1 a to IA-1c 
wherein R1a is H, CH3, C2H5 or nxe2x80x94C3H7, and alkyl is an alkyl group having from 1 to 8 carbon atoms.
Preferred compounds of the formula IA-2 are those selected from the group consisting of IA-2a to IA-2c: 
wherein R1a is H, CH3, C2H5 or nxe2x80x94C3H7, and alkyl is an alkyl group having from 1 to 8 carbon atoms.
The mixtures according to the invention preferably comprise one or more compounds of the formulae IA-1a and/or IA-2a.
Further preferred compounds in the mixtures according to the invention are those of the formulae IA-1b, IA-1c, IA-2b and IA-2c in which R1a is H.
The media according to the invention preferably comprise one or more compounds of the formula IA-1 and/or one or more compounds of the formula IA-2, in particular those of the preferred sub-formulae mentioned above.
The medium according to the invention preferably comprises one or more compounds of the formulae selected from the group consisting of IB-1 to IB-7: 
wherein Rb is as defined above.
Of the sub-formulae, preference is given to compounds in which Rb is a straight-chain alkyl radical having 1-8 carbon atoms or a straight-chain alkenyl radical having 2-8 carbon atoms.
The medium according to the invention preferably comprises one, two, three or four, preferably three or four, compounds of the formulae IB-1, IB-2, IB-3 and/or IB-4.
Preferred compounds of formula IB-4 are, 
Preferred compounds of the formula IC are those in which Rc is alkenyl having from 2 to 7 carbon atoms, in particular compounds selected from the formulae IC-1a to IC-1e: 
wherein R3a and R4a are each, independently of one another, H, CH3, C2H5 or nxe2x80x94C3H7.
Preference is given to compounds of the formula IC-1a, in particular those in which R3a and R4a are CH3, and to compounds of the formula IC-1e in which R3a is H.
Preference is given to TN and STN displays according to the invention in which the liquid-crystal mixture comprises at least one compound of the formulae IC-1a and/or IC-1c in which R3a and R4a each have the same meaning, and to displays in which the liquid-crystal mixture comprises at least one compound of the formula IC-1e.
In a further preferred embodiment, the mixtures according to the invention comprise one or more compounds of the formulae IC-2 and/or IC-3: 
wherein Rc and Rd are as defined above.
Preference is given to compounds of formulae IC-2 and IC-3 wherein Rd is F, OCF3 or alkyl having from 1 to 8, in particular 1, 2 or 3, carbon atoms, and Rc is 1E-alkenyl or 3E-alkenyl having from 2 to 7, in particular 2, 3 or 4, carbon atoms, and to compounds selected from formulae IC-2a to IC-2d and IC-3a: 
wherein R3a and R4a are each, independently of one another, H, CH3, C2H5 or nxe2x80x94C3H7, and alkyl is an alkyl group having from 1 to 8 carbon atoms.
Besides the compounds of the formulae IA, IB and IC, the mixtures according to the invention preferably comprise compounds of the formulae II and/or III 
wherein
R2 and R3 are each, independently of one another, an alkyl, alkoxy or alkenyl group having up to 12 carbon atoms, in which, in addition, one or two non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94COxe2x80x94Oxe2x80x94 in such a way that O atoms are not linked directly to one another, 
L1 to L4 are each, independently of one another, H or F,
Z1 and Z2 are each, independently of one another, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94C2F4xe2x80x94 or a single bond,
b and c are each, independently of one another, 0 or 1,
Q is xe2x80x94CF2xe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94CFHxe2x80x94, xe2x80x94OCFHxe2x80x94 or a single bond, and
Y is F or Cl.
Preferred compounds of formula III are those in which L1 and/or L2 are F and Qxe2x80x94Y is F, OCF2H or OCF3. Preference is furthermore given to compounds of the formula III in which R3 is 1E-alkenyl or 3E-alkenyl having from 2 to 7, in particular 2, 3 or 4, carbon atoms.
Preferred compounds of the formula II are the cyano compounds of the formulae IIa to IIi 
wherein R2 is as defined above. R2 in these compounds is preferably alkyl or alkoxy having from 1 to 8 carbon atoms.
Preference is given to mixtures which comprise one or more compounds of formulae IIa, IIb and IIg.
Preferred compounds of the formula III are compounds of formulae IIIa to IIIt 
wherein R3 is as defined above, and L1 and L2 are each, independently of one another, H or F. R3 in these compounds is preferably alkyl or alkoxy having from 1 to 8 carbon atoms.
Qxe2x80x94Y is preferably F, Cl, OCF3 or OCHF2.
Media according to the invention additionally preferably comprise compounds of formulae IIIa, IIIb, IIIc, IIId, IIIe, IIIf, IIIh, IIIi, IIIj, IIIk, IIIm, IIIn, IIIp and IIIt.
In the compounds of formulae IIIa to IIIt, Qxe2x80x94Y is preferably F, OCF3 or OCHF2, furthermore Cl.
The individual compounds of formulae II and III and their sub-formulae and other compounds which can be used in the displays according to the invention, preferably TN and STN displays, are either known or can be prepared analogously to the known compounds.
The compounds of formulae II and III are to be assigned to component A.
Besides the compounds of formulae IA, IB and IC, the mixtures according to the invention preferably furthermore comprise one or more liquid-crystalline tolan compounds. Because of the high birefringence (xcex94n) of the tolan compounds, it is possible to use smaller layer thicknesses, thus significantly shortening the response times. The tolan compounds are preferably selected from group T containing compounds of formulae T1, T2 and/or T3: 
d is 0 or 1,
L1 to L7 are each, independently of one another, H or F,
Q is xe2x80x94CF2xe2x80x94, xe2x80x94CHFxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94OCHFxe2x80x94 or a single bond,
Y is F or Cl,
Z4 is xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94CH2CH2xe2x80x94 or a single bond,
R4 and R5 are each, independently of one another, an alkyl group having up to 12 carbon atoms which is unsubstituted or substituted by at least one halogen atom and in which one or two non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94COxe2x80x94Oxe2x80x94 in such a way that O atoms are not linked directly to one another, where, in the case where d=1, R5 may alternatively be F, Cl, CF3 or OCF3.
Preferred compounds of formula T1 are of sub-formulae T1a and T1b 
wherein L1 to L4 and Qxe2x80x94Y are as defined above.
Preference is given to compounds of formula T1b-1 
wherein R4 is as defined above.
Preferred compounds of formula T2 are of sub-formulae T2a to T2h 
wherein R4, R5 and Z4 are as defined above, and L1 to L6 are each, independently of one another, H or F.
Preferred compounds of the formula T3 are of sub-formulae T3a and T3b 
Preference is given to compounds of formulae T2a and T2b.
In a further preferred embodiment, the mixtures comprise one or more compounds of the formula T2h.
Preferred compounds of formula T2e are those in which one, two or three of the radicals L1 to L6 are F and the others are H, where L1 and L2 or L3 and L4 or L5 and L6 are not both simultaneously F.
The proportion of the compounds from the group consisting of T2a and T2b is preferably from 5 to 50%, in particular from 10 to 40%.
The proportion of the compounds of formula T2h is preferably from 2 to 35%, in particular from 4 to 25%.
The proportion of the compounds of formula T1b-1 is preferably from 2 to 25%, in particular from 4 to 15%.
The proportion of the compounds from group T is preferably from 2 to 55%, in particular from 5 to 35%.
Preferred liquid-crystal mixtures comprise one or more compounds of component A, preferably in a proportion of from 15% to 80%, particularly preferably from 20% to 70%. These compounds have a dielectric anisotropy xcex94xcex5 of  greater than 1.5, preferably xe2x89xa73, in particular xcex94xcex5 greater than 8, particularly preferably xcex94xcex5xe2x89xa712.
Preferred liquid-crystal mixtures comprise one or more compounds of component B, preferably in a proportion of from 20 to 85%, preferably in a proportion of from 30 to 75%. The compounds of component B, in particular those containing alkenyl groups, are distinguished, in particular, by their low values for the rotational viscosity xcex31.
Compounds of formula IC are to be assigned to component B.
The mixtures according to the invention preferably comprise one or more compounds selected from the group consisting of the bicyclic compounds of formulae 
and/or one or more compounds selected from the group consisting of the tricyclic compounds of the following formulae: 
and/or one or more compounds selected from the group consisting of the tetracyclic compounds of formulae 
wherein R6 and R7 are as defined for Ra, L is H or F, and the 1,4-phenylene groups in IV10 to IV19 and IV23 to IV26 may each, independently of one another, also be monosubstituted or polysubstituted by fluorine.
R6 and R7 in the compounds of the formulae IV1 to IV35 are preferably straight-chain alkyl or alkoxy having 1 to 12 carbon atoms.
The compounds of formulae IV1 to IV33 are all to be assigned to component C.
The liquid-crystalline mixtures optionally comprise an optically active component C in an amount such that the ratio between the layer thickness and the natural pitch of the chiral nematic liquid-crystal mixture is greater than about 0.2, preferably 0.2 to 1.3. A multiplicity of chiral dopants, in some cases commercially, are available to the person skilled in the art for the component, such as, for example, cholesteryl nonanoate, S-811, from Merck KGaA, Darmstadt, and CB15 (BDH, Poole, UK). The choice of the dopant or dopants is not crucial per se.
The proportion of the compounds of component C is preferably from 0 to 10%, in particular from 0 to 5%, preferably from 0 to 3%.
The mixtures according to the invention may also optionally comprise up to 20% of one or more compounds having a dielectric anisotropy of less than xe2x88x921.5 (component D).
If the mixtures comprise compounds of component D, these are preferably one or more compounds containing the structural unit 2,3-difluoro-1,4-phenylene, for example, compounds as described in DE-A 38 07 801, 38 07 861, 38 07 863, 38 07 864 and 38 07 908. Preference is given to tolans containing this structural unit as described in the International Patent Application PCT/DE 88/00133.
Further known compounds of component D are, for example, derivatives of the 2,3-dicyanohydroquinones, or cyclohexane derivatives containing the structural unit 
as described in DE-A 32 31 707 and DE-A 34 07 013.
The liquid-crystal displays according to the invention preferably comprise no compounds of component D.
The term xe2x80x9calkenylxe2x80x9d in the definitions of Ra, Rb, Rc, and R1-R7 covers straight-chain and branched alkenyl groups, preferably having 2-7 carbon atoms, in particular the straight-chain groups. Preferred alkenyl groups are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl and C7-6-alkenyl, in particular C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4-alkenyl.
Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are generally preferred.
In preferred embodiments, the mixtures comprise
from 5 to 30% by weight, in particular from 8 to 20%, of one or more compounds of formula IA;
from 10 to 40% by weight, in particular from 10 to 30% by weight, of one or more compounds of formula IB;
from 25 to 60% by weight, in particular from 25 to 50%, of one or more compounds of formula IC;
one or more, preferably one, two or three, tolan compounds of formula T2h 
xe2x80x83wherein R4 and R5 are as defined above;
one or more, preferably in each case from two to four, tolan compounds of the following formulae: 
xe2x80x83wherein R4 and R5 are as defined above;
one or more compounds of the following formulae: 
xe2x80x83wherein R6, R7 and L are as defined above. L in formula IV27 is preferably F;
one or more, in particular from two to five, compounds selected from the group consisting of the compounds IIIa to IIIg;
at least two compounds selected from the following group consisting of the compounds 
xe2x80x83wherein alkyl is an alkyl group having 1 to 8 carbon atoms, and R3a is H, CH3, C2H5 or nxe2x80x94C3H7;
more than 20% of compounds having positive dielectric anisotropy, in particular having xcex94xcex5xe2x89xa7+12.
The mixtures according to the invention are distinguished, in particular on use in TN and STN displays of high layer thicknesses, by very low total response times (ttot=ton+toff).
The liquid-crystal mixtures used in the TN and STN cells according to the invention are dielectrically positive, with xcex94xcex5xe2x89xa71. Preference is given to liquid-crystal mixtures with xcex94xcex5xe2x89xa73, in particular with xcex94xcex5xe2x89xa75.
The liquid-crystal mixtures according to the invention have favorable values for the threshold voltage V10/02/20 and for the rotational viscosity xcex31. If the value for the optical path difference dxc2x7xcex94n is prespecified, the value for the layer thickness d is determined by the optical anisotropy xcex94n. In particular at relatively high values for dxc2x7xcex94n, the use of liquid-crystal mixtures according to the invention that have a relatively high value for the optical anisotropy is generally preferred, since the value for d can then be selected to be relatively small, which results in more favorable values for the response times. However, liquid-crystal displays according to the invention which contain liquid-crystal mixtures according to the invention with smaller values for xcex94n are also characterised by advantageous values for the response times.
The liquid-crystal mixtures according to the invention are furthermore characterised by advantageous values for the steepness of the electro-optical characteristic line, and can be operated with high multiplex rates, in particular at temperatures above 20xc2x0 C. In addition, the liquid-crystal mixtures according to the invention have high stability and favorable values for the electrical resistance and the frequency dependence of the threshold voltage. The liquid-crystal displays according to the invention have a large working-temperature range and good angle dependence of the contrast.
The construction of the liquid-crystal display elements according to the invention from polarisers, electrode base plates and electrodes having a surface treatment such that the preferential alignment (director) of the liquid-crystal molecules in each case adjacent thereto is usually twisted by a value of from 160xc2x0 to 720xc2x0 from one electrode to the other corresponds to the usual structure for display elements of this type. The term xe2x80x9cusual structurexe2x80x9d here is broadly drawn and also covers all derivatives and modifications of the TN and STN cell, in particular also matrix display elements and display elements containing additional magnets.
The surface tilt angle at the two outer plates may be identical or different. Identical tilt angles are preferred. Preferred TN displays have pre-tilt angles between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0xc2x0 to 7xc2x0, preferably from 0.01xc2x0 to 5xc2x0 in particular from 0.1xc2x0 to 2xc2x0. In the STN displays, the pre-tilt angle is from 1xc2x0 to 30xc2x0, preferably from 1xc2x0 to 12xc2x0 and in particular from 3xc2x0 to 10xc2x0.
The twist angle of the TN mixture in the cell has a value of between 22.5xc2x0 and 170xc2x0, preferably between 45xc2x0 and 130xc2x0 and in particular between 80xc2x0 and 115xc2x0. The twist angle of the STN mixture in the cell from alignment layer to alignment layer has a value of between 100xc2x0 and 600xc2x0, preferably between 170xc2x0 and 300xc2x0 and in particular between 180xc2x0 and 270xc2x0.
The liquid-crystal mixtures which can be used in accordance with the invention are prepared in a manner which is conventional per se. In general, the desired amount of the components used in lesser amount are dissolved in the components making up the principal constituent, advantageously at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example in acetone, chloroform or methanol, and to remove the solvent again, for example, by distillation, after thorough mixing.
The dielectrics may also comprise further additives which are known to the person skilled in the art and are described in prior art literature. For example, 0-15% of pleochroic dyes may be added.
In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by means of acronyms, the transformation into chemical formulae taking place in accordance with Tables A and B. All radicals CnH2n+1 and CmH2m+1 are straight-chain alkyl radicals having n and m carbon atoms respectively (n and m=1-12). The alkenyl radicals have the trans-configuration. The coding in Table B is self-evident. In Table A, only the acronym for the parent structure is indicated.
In individual cases, the acronym for the parent structure is followed, separated by a dash, by the code indicated in the table below for the substituents R1*, R2*, L1, L2 and L3.
The mixtures according to the invention preferably comprise one or more compounds from Tables A and B.